A surface acoustic wave (SAW) device has been known conventionally as one of the devices adopting an acoustic wave. The SAW device is used in various circuits that process radio signals in a frequency band of 45 MHz to 2 GHz typically in mobile telephones, for example. Examples of various circuits include a transmitting band-pass filter, a receiving band-pass filter, a local filter, an antenna duplexer, an IF filter, and an FM modulator.
Recently, a boundary acoustic waves device using boundary acoustic waves have been developed. International Publication No. WO98/52279 discloses a boundary acoustic waves device.
However, the above-mentioned boundary acoustic waves device has a problem that acoustic waves to be excited are generated at the boundary between a first medium and a second medium, for example, in addition to the boundary between a piezoelectric substrate and the first medium, and the acoustic wave influences the characteristics as an unnecessary response.